Silicone coating compositions are useful for many applications including release (adhesive) coatings, protective coatings, and conformal coatings. These coatings are usually applied to substrates as dispersions in a solvent system in order to reduce the viscosity sufficiently so that the coating composition is easily coatable. The presence of the solvent, either water or some suitable low boiling organic solvent, necessitates evaporation. Thus, the application of heat to articles coated with silicones has served two purposes, removal of solvent and thermally induced curing. The elimination of solvent is desirable for two significant reasons some organic solvents constitute an environmental and/or a safety hazard and elimination of evaporation reduces the energy requirements for preparation of the coated article. Elimination of the need for a heat curing step allows consideration of alternative curing mechanisms such as a radiation cure employing either actinic or electron beam radiation.
The absence of solvent from a coating composition lowers the necessary energy required for cure eliminating the need for costly pollution abatement equipment. This absence of solvent has a drawback insofar as some silicone coating compositions are thick, viscous mixtures that are difficult to coat without the viscosity reduction provided by the dilution of a solvent. Thus thin, defect-free, and uniform coatings are difficult to achieve with such materials. Certain photo-catalysts, particularly onium type photo-catalysts, may require the presence of a compatibilizing solvent to render them soluble in the silicone coating mixture.
Radiation curable protective coatings are useful for a variety of applications, such as topcoats over printed matter and similar uses. Such radiation curable compositions are typically highly functional reactive substances which cure to yield highly cross-linked, glossy, and hard coatings. Silicone coatings of this type frequently contain so-called Q functional siloxane groups (SiO.sub.4/2) in their structure. Such Q containing silicones are generally referred to as resins to distinguish them from silicone fluids which are usually linear, consisting of largely of repeating D groups (R.sub.2 SiO.sub.2/2, where R usually is methyl). Highly reactive organofunctional silicone Q resins are typically very viscous semi-solids or friable solids at room temperature and thus not readily coatable because they are not liquid. Indeed, when functionalized with polar organic moieties that are radiation cross-linkable, even linear silicone fluids can become highly viscous and therefore difficult to coat using standard coating techniques.
Because of their low durometer and extensible nature when cured, silicone conformal coatings are useful for the protection of electrical components and circuitry. Fluorosilicones are particularly well-suited to use as conformal coatings because of their resistance to solvent swelling and degradation. Photo-curable silicones that are intended for low modulus cured coatings typically consist of linear silicone molecules that have photo-reactive centers widely separated by non-functional polysiloxane segments so that a low cross-link density results when the silicone is cured. Because they consist almost entirely of polydimethylsiloxane segments, such photo-curable silicone polymers are incompatible with 'onium ionic photo-catalysts. This incompatibility results in an inefficient and/or slow photo-cure. The fluorosilicone polymers suffer from these same drawbacks.
Silicone compositions have long been used for rendering surfaces non-adherent to materials which would normally adhere thereto. Epoxy-functional silicones such as taught in U.S. Pat. No. 4,279,717, when combined with certain compatible iodonium cationic photo-catalysts, are known to be useful for release coating purposes. Epoxy-silicone release coatings allow high speed processing with minimal energy expenditures. If the viscosity of the coating composition exceeds 1,000 centistokes (cstk) at room temperature, the absence of solvent in the composition renders them difficult to apply, particularly if a thin coating on the order of 1 gm/m.sup.2 is desired. The viscosity constraint imposed by processing equipment thus imposes constraints on the molecular weight of the silicone composition and on linearly functionalized photo-curable silicone fluids such as epoxy-silicones. Additional constraints are provided by the need for photo-catalyst miscibility or solubility, the need for a rapid photo-cure response, and good release performance. While a high epoxy content in an epoxy-silicone, as epoxy functional groups on a linear silicone molecule, tend to promote 'onium photo-catalyst compatibility with the silicone and a rapid photo-cure, a low epoxy content is required for premium or low force release characteristics.
Controlled release is an additional aspect of photo-curable epoxy-silicone release performance. Compositions containing both epoxy functional and phenol functional silicones as taught in U.S. Pat. No. 5,138,012, and Q resins containing epoxy-silicones as taught in U.S. Pat. Nos. 5,360,833 and 5,369,205 provide a so-called controlled release. Controlled release refers to a controllable and predictable release force that may be varied from very easy to very tight depending on the desired application.